Solid state radiation sensitive field electron emitter and methods of fabrication thereof

ABSTRACT

A solid state radiation sensitive field emitter cathode comprising a single crystal semiconductor member having a body portion with a uniform array of closely spaced and very sharp electron emitting projections from one surface in the form of needles or whisker like members. Electrons are emitted into vacuum when a planar-parallel positive anode is mounted in close proximity to the surface. The cathode is responsive to input radiation such as electrons or light directed onto the cathode in modifying the electron emission from the array of electron emitter projections. The method of manufacturing the cathode by providing a predetermined pattern or mosaic of islands of a material exhibiting a greater etch resistant property than the semiconductor material, on a wafer of a semiconductor material and then etching out between and beneath the islands to undercut to a point where the islands are supported by only a small whisker of the semiconductor material. Removal of the islands results in an electron emitter being exposed from beneath each island wherein carriers generated within the body portion and also carriers generated within the depletion regions of the tips diffuse to the electron emitter projections wherein establishment of a high electric field at the tips of the electron emitter projections results in electron emission primarily due to conduction band tunneling. The device provides about 106 emitting points of close proximity so as to effect photographic-like imaging.

United States Patent [191 Nathanson et al.

SOLID STATE RADIATION SENSITIVE FIELD ELECTRON EMITTER AND METHODS OFFABRICATION THEREOF [75] Inventors: Harvey C. Nathanson, Pittsburgh;

Richard N. Thomas, Murrysville; Jens Guldberg, Penn Hills, all of Pa.

[73] Assignee: Joseph Lucas, (lndustries) Limited,

Birmingham, England [22] Filed: Feb. 11, 1972 [211 App]. No.: 225,517

[52] US. Cl 313/95, 313/96, 313/105, 313/309, 313/351 [51] Int. Cl H01j39/06, HOlj 39/16 [58] Field of Search 313/94, 95, 96, 103, 104,

[56] References Cited UNITED STATES PATENTS 3,466,485 9/1969 Arthur, Jr.et al 3l3/95 3,665,24l 5/]972 Spindt et al 313/309 X PrimaryExaminer-Herman Karl Saalbach Assistant Examiner-Siegfried H. GrimmAttorney, Agent, or FirmF. H. Henson [57] ABSTRACT A solid stateradiation sensitive field emitter cathode comprising a single crystalsemiconductor member having a body portion with a uniform array ofclosely spaced and very sharp electron emitting projections from onesurface in the form of needles or whisker like members. Electrons areemitted into vacuum when a planar-parallel positive anode is mounted inclose proximity to the surface. The cathode is responsive to inputradiation such as electrons or light directed onto the cathode inmodifying the electron emission from the array of electron emitterprojections. The method of manufacturing the cathode by providing apredetermined pattern or mosaic of islands of a material exhibiting agreater etch resistant property than the semiconductor material, on awafer of a semiconductor material and then etching'out between andbeneath the islands to undercut to a point where the islands aresupported by only a small whisker of the semiconductor material. Removalof the islands results in an electron emitter being exposed from beneatheach island wherein carriers generated within the body portion and alsocarriers generated within the depletion regions of the tips diffuse tothe electron emitter projections wherein establishment of a highelectric field at the tips of the electron emitter projections resultsin electron emission primarily due to conduction band tunneling. Thedevice provides about 10 emitting points of close proximity so as toeffect photographic-like imaging.

13 Claims, 22 Drawing Figures 2.5no' cm +4 PATENTEDJUH 4 1914 smunr?Pm'mmm 4mm 33141968.

SHEET 2 0f 7 CONDUCTION BAND TUNNELI N6 PA TENTEU 4 I974 QUANTUM YIELDELECTRONS/ INCIDENT PHOTON SHEET 0F 7 VFIG.I7

WAVELENGTH MICRONS PATENTEBJuu 4:914

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PA'TB'NTEDJUM 4 m4 J-AMPS d-AMPS $814,968 sum 6 0F 7 IO OHM CM PTYPE(III) ANODE SPACING .508 cm FIG. I9

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I60 OHM CM P TYPE (III) I I l l l I .OOOOI .OOOI .OOI .Ol .I I

FILTER TRANSMISSION INPUT LIGHT FLUX, ARBITRARY UNITS SOLID STATERADIATION SENSITIVE FIELD ELECTRON EMITTER AND METHODS OF FABRICATIONTHEREOF BACKGROUND OF THE INVENTION This invention relates generally tocold cathode field electron emitters and more particularly to those ofthe type where the emitter is responsive to input radiation such aselectrons, X-rays or light and particularly radiations in the infraredregions.

Photoemissive type devices are well known in the art and the most commonare the tri-alkali antimonides These structures are sensitive in thevisible spectral region but lack sensitivity in the infrared region. Inthe last few years, a new type of photoemitter known as the III-Vcompound .semiconductor has provided improved sensitivity in theinfrared region. These photoemitters rely on the use of monolayercoatings of Cs and C 0. They suffer disadvantages of requiring hightemperature processing and very good operating vacuum. Silicon pointarrays have neither of these disadvantages. Field emission type cathodesin which emission occurs in response to an intense electric field arealso known. In the last few years work has been done utilizingsemiconductors such as silicon with a plurality of whiskers providedthereon and which is sensitive to light. Such a device is described inUS. Pat. No. 3,466,485 by John R. Arthur, et al. Experimental results onsingle emitter tips are also reported in an article entitled.Photo-Field-Emission from High Resistance Silicon and Germanium by P.G. Borzyak, et al, on page 403, Phys. Stat. Sol. 14,403 (1966). Thephoto-field-emitting type emitter provides a very sensitive typedevice.- The Arthur, et al, arrays are made using a vapourliquid-solidgrowth mechanism using gold to seed whisker growth. As a result, theArthur, et al, device has thewhisker points saturated with gold which isa most effective lifetime-killer. For good photoresponse, high lifetimeis essential Because of poor lifetime, the light in the Arthur, et al,device must be directed onto the very tip of the semiconducting points.The prior art teaching has thus failed to recognize or fabricate adevice capable of utilizing its full potential. The prior art activityappears to have been mostly in the area of experimentation and hasfailed to 1 bring forth a device for commercial utilization. The presentinvention is directed to improved structures and fabrication methods toaccomplish improvements over the prior art teaching.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided a radiation-sensitive field-emissive cathode whereinan-array of electron emitting projections is provided on one surface ofa wafer of a suitable semiconductive material such as P-type silicon ofohm cm and greater resistivity. Radiation is directed onto the waferfrom theopposite surface with respect to the electron emitter array andcarriers are generated within about a 100 micrometers of the emittertips and diffuse to the emitter tips where they are emitted into thevacuum. The method'of fabricating the electron emitter array on a waferof a semiconductive material utilizes photoresist techniques todelineate a predetermined pattern .or mosaic of islands ofetch-resistant material on the surface of the wafer, followed by etchingaway the material of the wafer between and beneath the islands untilonly a needle-like projection member remains below each island. Theseislands may then be removed to expose a semiconductive emitting devicewith an array of needle-like projections formed of the original wafer.This fabrication technique retains the high crystalline perfection ofthe starting semiconductor material within the structure to ensure thathigh carrier lifetime is maintained and thereby providing long diffusionpaths (about 25 to 250 micrometers) for the carriers. Alsonoredistribution of impurities can occur in this room temperatureprocess. The improved structure also provides surface regions andcoating on surfaces of the semiconductor wafer to enhance sensitivity byincreasing absorption of input radiation, reduced loss of inputradiation produces generated carriers and minimizes dark currentgeneration.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of theinvention, reference may be had to the preferred embodiments, exemplaryof the invention, shown in the accompanying drawings, in which:

FIG. 1 is a schematic view of an image intensifier incorporating aphotocathode in accordance with the teachings of this invention;

FIG. 2 is a schematic showing of a camera tube incorporating aphotocathode and an electron multiplier in accordance with the teachingsof this invention;

7 FIG. 3 is a perspective view of the photocathode in FIGS. 1 and 2illustrating the electron emitter array.

FIG. 4 is a side view of one of electron emitting projections of thearray illustrated in FIG. 3;

FIGS. 5, 6, 7, 8 and 9 illustrate steps in the manufacture of thephotocathode illustrated in FIGS. 1 and 2;

FIG. 10 illustrates another embodiment of the electron emittingprojection array cathode for use in FIGS. 1 and2;

FIGS. ll, 12 and I3 illustrate steps in the manufacture of the deviceshown in FIG. I0;

FIG. 14 illustrates a further embodiment of the invention illustratingan electron emitter projection array cathode for use in FIGS. 1 and 2;

FIGS. 15A and 15B illustrate respectively an electron emitter projectionof the present invention, and an energy band diagram, which facilitatesunderstanding the mechanism of operation of the electron emitters of thepresent invention.

FIG. 16 illustrates an experimental Fowler-Nordheim plot of log J. V. INanode for P-type silicon, 10 ohm cm(lll)-emitter array at roomtemperature for the photocathode shown in FIG. 1;

FIG. 17 illustrates the response of the prior art structures incomparison with the silicon field emitter array;

FIG. 18 illustrates the emission characteristics of 160 ohm cm p-type ll l) silicon field emitter array at K in the dark and at differentintensities of 1.06 micron radiation inputs;

FIG. 19 illustrates the dark emission characteristics of a 10 ohm cmp-type l l l) silicon field emitter array at different temperatures;

. FIG. 20 is an Arrhenius plot of data shown in FIG. 19, and

FIG. 21 illustrates a plot of a photocathode response to input light.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1, 3 and 4,there is illustrated an image intensifier device including an evacuatedenvelope having an input window 18 and an output window 24. Aphotocathode 12 in accordance with the teachings of this invention isprovided on the inner surface of the input window 18. Suitable coolingmeans 19 is provided about the photocathode 12 to control thetemperature thereof. It may be necessary in some applications to reducethe dark current due to thermal generation. An output screen 14 isprovided on the inner surface of the output window 24. The screen 14comprises a layer 20 of suitable phosphor material which emits radiationin response to electron bombardment. An electrical conductive coating 22of a suitable material such as aluminum may be provided on the innersurface of the layer 20. The layer 22 provides not only an electricalconnection but is also opaque to and prevents radiation from thephosphor screen 20 being directed back on to the photocathode 12.

An extractor grid electrode 27 is provided between the photocathode 12and the output screen 14. The electrode 27 is a mesh of electricallyconductive material and about 50 to 80 per cent transmissive. Theextractor electrode 27 is also sufficiently rigid to prevent distortiondue to electrical fields. The extractor electrode 27 should be spacedsuch a distance as to provide adequate electrical field to cause fieldemission from the photocathode 12. The electrode 27 may be spaced at adistance of about 250 micrometers from the photocathode 12. A suitablepotential provided by a battery 26 of about 5,000 volts is connectedbetween the photocathode l2 and the extractor electrode 27. Theelectrical contact to the photocathode 12 is made by means of a P+region 15. The output screen 14 may be positioned at a distance of about250 micrometers from the extractor electrode 27 to provide a variableaccelerating potential to accelerate the electrons to the output screen14. A variable potential source 29 is connected between the outputscreen 14 and the extractor electrode 27. The variable potential source29 provides means of varying the output brightness of the device. Theextractor electrode 27 may be omitted in some applications and theoutput electrode 14 will provide a simultaneous extraction andacceleration potential.

The photocathode 12 is of a suitable semiconductor material such as asilicon, germanium, III-V compounds and ternary Ill-V compoundsemiconductors. Elemental semiconductors with deep impurity levels suchas, silicon doped with gold could also be used. The specific embodimentutilizes P-type silicon material having a resistivity of 0.1 to 160 ohmcm. The photocathode 12 is fabricated from a single crystal wafer. Thephotocathode 12 comprises a body portion 11 having a thickness of about25 micrometers. An array 16 of a plurality of projections 13 projectsfrom the body portion 11 to a height of about 12 micrometers. Thespacing between the emitting projections 13 may be about 25 micrometers,with the substrate thickness being less than about 500 micrometers, andthe diameter of the tip of the projections 13 may be less than 1micrometer. In the specific device, the diameter of the tips was about0.5 micrometers. The base of the emitting projection 13 may be about 25micrometers. The P+ region 15 is provided on surface of the body portion11 remote to the array 16. A photocathode of 3.2 cm in diameter may haveas many as 1.25 X 10 projection emitters 13.

The general principles of operation of this device are best'understoodwith reference to FIGS. 15A, 15B, l6, 18, 19, 20 and 21. Electrons areemitted from the sharp point or tip of the emitter projections 13 whenplaced in close proximity to a positively biased extractor electrode 27as shown in FIG. 15A, due to the field intensification at the tip. Atlow applied extractor voltages, electrons are emitted from theconduction band of the silicon. The emission is well-described by theFowler- Nordheim tunneling theory which results in a linear log currentvs inverse voltage relationship as shown in region l of FIG. 16. Athigher applied anode voltages, which are sufficient to overcomeshielding effects of surface charge states, penetration of the electricfield into the semiconductor tip can occur. A space-charge region whichis essentially depleted of mobile carriers is therefore created at thetip of the emitter 13. As a result, it is observed that the log currentvs inverse voltage plot assumes a lesser slope as shown in region 2 ofFIG. 16, since the supply of electrons in the conduction band at thesurface available for emission is limited. In this source-limited modeof operation, the device is sensitive to input radiation, such asphotons or incident electrons, which alters the electron population inthe conduction band'and thereby increases the emission current. Indetail, operation thus depends upon the formation of electron-hole pairby the input radiation within the space-charge region at the tip of theemitter 13 and/or within a diffusion length in the bulk p-region of theemitter 13 and body 11. Electrons generated within this bulk p-regiondiffuse into the space-charge region. Because of the built-in electricfield within the space-charge region, electrons created or diffusinginto this region, are therefore swept to the tip of the emitter 13 fromwhence they are emitted. In the case of input radiation consisting ofphotons, each absorbed photon generates an average of one electron-holepair. With electrons incident on the device however, each incidentelectron creates one electron-hole pair per 3.5 eV energy on theaverage. Substantial electron gains are therefore possible withenergetic electrons, e.g. IOKeV electrons result in gains of 2,000.

The dark current emission characteristics shown in FIG. 19 is of anunpassivated photocathode and indicates that the dark current issubstantially reduced at operating temperatures below room temperature.Furthermore, the dependence of dark current of the device shown in FIG.20 yields an activation energy of 0.5 6eV. This value of activationenergy which is equal to onehalf of the band gap energy of siliconindicates unambiguously that terminal generation via mid-gap bulk andsurface states is the source of dark current in the device. Thus,substantial reductions in the dark current are possible with theapplication of well-known oxide passivation and gettering techniques tothe device.

In FIG. 18, a family of curves for different values of light at awavelength of 1.06 microns is shown with the temperature held at K.

The extended source-limited behaviour observed at the lowest lightlevels and the fact that the photocurrent remains approximately constantover a range of applied anode voltages is of considerable interest.During fabrication, small variations in tip diameter and height of theemitter projections 13 within an array 16 may occur. Small variations inthe electric field at the tips of individual emitter 13 would thereforeoccur when this array is operated at constant anode voltage. However,due to the relative insensitivity of the photocurrent to changes in theelectric field in the sourcecontrolled mode, the emitted photocurrentfrom each emitter should be constant. In other words, certainnonuniformities of the point emitter dimensions can be tolerated withoutaffecting the photoemission uniformity.

In FIG. 21 the linearity between photocurrent and illumination levelover five orders of magnitude is shown and corresponds to y 1.Saturation is determined by the eventual changeover to tunneling-limitedemission at high light-flux levels. The light level at which saturationoccurs can be preset by proper choice of the applied anode voltage, andcan be used to minimize blooming of bright elements in an otherwisedimly illuminated scene.

In FIG. 2, there is illustrated a pick-up tube which again utilizes thephotocathode 12 as described with respect to FIGS. 1, 3 and 4. Thepick-up tube comprises an evacuated envelope 30 having an input window18 of a suitable material transmissive to input radiation such asborosilicate glass or quartz. The photocathode is disposed thereon.Suitable cooling means 19 is also provided.- The electron emitter array16 of the photocathode 12 is remote to the input window 18 and theelectron emitting array 16 faces an electron multiplying electrode 33.The electrode 33 may be fabricated in same manner and of similarstructure as the photocathode 12. The photocathode 12 may be operated ata potential of about 10,000 volts negative with respect to ground and isprovided by a suitable potential source 32. The spacing between themultiplier electrode 33 and photocathode 12 may be about 250micrometers. The potential on the electrode 33 may be about 5,000 voltsnegative with respect to ground and is provided by a suitable potentialsource 34. The multiplier electrode 33 is identical to the photocathodel2 and is responsive to electron bombardment. With the potentials shown,one may obtain an output of about 500 electrons from the emitting arraysurface of the electrode 33. in response to each incident electronemittedfrom the photocathode 12. A target electrode 39 is providedadjacent the emitting array surface of the electrode 33 and may be ofany suitable target material which exhibits the property of storage ofcharge in response to electron bombardment. The target 39 may be of "anysuitable type such as described in US. Pat. No. 3,440,476 by M. H.Crowell or of the type in US. Pat. No. 3,213,316 by G. Goetze, et al.

An electron gun 36 is provided at the opposite end of the envelope withrespect to the target structure 39 and directs a scanning electron beamover the target member 39 to read out the charge image in a wellknownmanner. This output signal is derived across an output resistor 38 ofthe target electrode 39. The electrode 39 may be operated at a potentialof about 10 volts positive with respect to ground by means of a battery41. The cathode of electron gun 36 may be operated at a potential ofabout ground.

Input radiations directed onto the photocathode l2 generate an electronimage corresponding to the input radiations. This electron image isaccelerated into the electrode 33 wherein the electron bombardmentgenerates charge carriers causing the field electron emission from theemitter array surface of the electrode 33. These electrons areaccelerated into incidence on the target electrode 39. By providing theelectrode 33 between the photocathode 12 and the target 39,amplification of the input signal is obtained. The target 39 providesthe necessary extraction potential for the electrode 33.

A method of fabricating the photocathode 12 or the electrode 33 isillustrated in FIGS. 5 through 9. The A figures are top view and the Bfigures are side views. A wafer 43 of a suitable p-type semiconductorsuch as silicon, germanium, gallium arsenicle or other III-Vsemiconductor compounds including tertiaries such asgallium-indium-arsenide and indium-arsenide phosphide and having bandgaps from 0.2 electron volt up to 3.0 electron volts may be utilized.The wafer 43 should be of a single crystal and have a suitable crystalorientation to provide the desired structure after etch. Crystalorientation of (111) and have been utilized. One specific example is a10 ohm centimeter ptype 1 l l silicon wafer having a thickness of about25 to 50 micrometers. The wafer 43 may be cut fromingots grown by theCsochralski or float-zone methods.

The first step in the operation is to oxidize the wafer 43 on onesurface to provide an oxide coating 42 as illustrated in FIG. 6. Thecoating 42 should be about I micrometer in thickness. The oxide coating42 may be provided by well-known techniques such as treating the waferin a wet oxygen atmosphere at a temperature at about 1,100C for about 2to 3 hours. The next step in the operation is to provide a photoresistmaterial coating on top of the silicon dioxide layer 42 and then exposewith radiation through an aperture mask and then remove the undesiredportions of the photoresist coating. The photoresist technique is wellknown in the art and one may spin on about .7 micrometers of a suitablephotoresist such as Positop and then expose with ultraviolet radiationand wash to provide a pattern of islands 41 of photoresist similar tothe pattern shown in FIG. 7. The silicon dioxide coating 42 is thenremoved from the uncovered regions by a suitable etch such as bufferedhydrofluoric acid etch (ammonium fluoride and hydrofluoric acid in 6:1proportions) and then the unsoluble photoresist is removed from theislands 41 to provide a pattern of silicon dioxide islands 41 asillustrated in FIG. 7. The islands 41 are circular and may have adiameter of about 20 micrometers and are spaced apart on centers byabout 25 micrometers.

The next step in the operation is a P+ diffusion into the back surfaceof the wafer 43. This is a well-known technique and may be accomplishedby exposing the wafer 43 to boron bromide BBr at 950C for a few minutes.The P+ layer 15 thus formed prevents or minimizes loss ofradiation-generated carriers at the back surface due to recombination aswell as provide an electrical contact. The next step in the operation isto rotate etch in a suitable etch such as 25 parts nitric acid, 10 partsacetic acid and one part hydrofluoric acid at 6.0 revolutions per minuteusing 50 cc of etch. This etch should be continued for about 20 minutesor until the tip dimensions of less than 0.5 micrometers have beenacheived. Etching is stopped by quenching with water followed by rinsingfirst in water and then in methanol. The PI- layer 15 on the backsurface is masked in this operation. Other suitable etchants for siliconare found in INTEGRATED SILICON DEVICE TECHNOLOGY, Volume X, ResearchTriangle Institute, Durham NC, November 1965. The resulting structure isshown in FIG. 8 and illustrates the formation of the array 16 ofprojections 13 on the body portion 11 of the photocathode. The silicondioxide islands 41 may then be removed by etching in bufferedhydrofluoric acid etch and then rinsing in water, methanol and thendrying.

An optional next step is to provide a passivating coating 9 of silicondioxide having a thickness of about 50 to 75 angstroms on the pointarray surface 16. The coating 9 may be formed by thermal growth in wetor dry oxygen at 600C for several hours. The structure is then subjectedto annealing in hydrogen at 350 to 450C for l to 2 hours. The wafer canthen be gettered. The completed photocathode 12 is then secured to theface plate 18. The resulting structure is shown in H0. 9. It is obviousthat the islands 41 may be formed by other methods and may be ofdifferent shapes.

The resulting structure provides a crystallographically continuousconnection between the body portion 11 and the projections 13. That isthe crystal perfection and low level of contaminating impurities iscontinuous throughout the body or substrate 11 and the projections 13.This is in contrast to prior art methods which incorporate processingtechniques that result in large concentrations of contaminatingimpurities in the tips of the projections. The high concentration of theimpurities are difficult to remove by standard gettering techniques.Because of the internal crystalline perfection and the smooth externalsurface resulting from the etch process, high effective lifetimes ofminority carriers in the body 11 and projections 13 accrue. Bulklifetimes of greater than I microsecond are obtained in the body 11 andprojections 13. The efficiency of collection of minority carriers ismuch greater than would accrue in prior art lll-V photoemitters. lll-Vphotoemitters require heavily doped P+ regions to maintain net surfacenegative electron affinity. Heavily doped P+ substrates usually havelifetimes in the nanoseconds regions, a factor of 1,000 below that ofthe present invention. Thus, a 30 fold increase in collection depth ofphotocreated minority carriers is afforded with subsequent increases ofefficiency of that order. It must be recognized that a high lifetime tipby itself would not provide adequate cross sectional area for efficientcreation of carriers due to input light images. It is the combination ofhigh lifetime emitting points crystallographically continuouslyfabricated on a high lifetime substrate region of significant thicknesswhich provides for the extremely high efficiencies of this invention.Carriers created deep within the body 11 are because of high internallifetime of the body capable of diffusing out to and along the length ofthe tip region where they are subsequently emitted. Thus, the efficiencyis limited only by resolution degradation associated with inordinatelythick targets.

lt is also possible to provide a self-supporting structure by startingwith a silicon wafer having a thickness of about 250 micrometers anddiameters of 3.2 cm and then etching out a central region of this waferof diameter of 2.5 cm to the desired thickness of about 25 micrometersand then proceeding with the steps illustrated in FIGS. through 9. Theresulting structure is a thin diaphragm of silicon with a supportingring thereabout having a thickness of about 250 micrometers. In thismanner the thin silicon wafer may be supported for utilization as atransmission type electron multiplier in which electrons are directedonto one surface and electrons are emitted from the opposite surfacethereof.

Referring to FIG. 10, another embodiment is shown. The photocathode issimilar to that previously described but included an oxide coating 55 onthe emitter array 16. The fabrication of the device is illustrated inFIGS. 11 and 13. A wafer 43 having a thickness of about 25 tomicrometers is provided. A coating 52 of silicon nitride SiN is providedon one surface of the wafer 43. The thickness of the coating is about0.2 micrometers and may be deposited by the ammonolysis of silane at700900C for about 20 minutes.

The next step is to provide a coating 54 of silicon dioxide of athickness of about 0.2 micrometers. The coating 54 may be thermallygrown by heating the wafer in dry or wet oxygen at l,l00C or may bedeposited by thermal decompositionof silane or oxygen at 600700C. Aphotoresist coating is then placed on the coating 54, exposed and amosaic of islands of photoresist of a similar pattern shown in HO. 7 isobtained.

The unprotected portion of the coating 54 is then removed by a suitableetch such as buffered hydrofluoric acid and then the unprotected coating52 is removed by etching in hot phosphoric acid to provide the structureshown in FIG. 1] and with the pattern similar to that shown in FIG. 7.At this stage the remaining oxide pattern can be removed if required.The silicon wafer 43 is then etched using a nitric, acetic andhydrofluoric acids mixture as previously described. Rather than etchingthe points down to about 0.5 micrometers, the etch is stopped when thepoint diameter is about 1.5 micrometers. This structure is shown in FIG.12. The next step is to provide the oxide coating 55 on the arraysurface. The oxide coating 55 is formed by thermal oxidation. The use ofthermal oxidation in forming the oxide coating 55 serves severaladvantageous functions in this embodiment. The thermal oxidation whichproceeds in a slow and well-controlled manner enables the silicon pointsto be reduced in diameter in a similar manner. If necessary, the tipdiameter could be trimmed to the required dimensions by repeatedoxidation and oxide-removal steps. In the specific device illustratedthe 1.5 micrometer diameter tip would be reduced to a 0.5 mm. tipintimately surrounded by 1.0 micrometer oxide coating. This is shown inFIG. 13. Upon removal of the nitride islands 52 the emitter pointsconsist of a clean 0.5 micrometer diameter silicon core surrounded by a1.0 mm. thick oxide coating. The oxide-coated array with islands inplace can be annealed in hydrogen at 450C. The nitride islands are thenremoved and the oxide coating +H anneal provides a means of passivatingthe surface to substantially reduce background dark current in thedevice. The final structure is illustrated in FIG. 10.

FIG. 14 illustrates another embodiment, prior to removal of the islands50 a layer 58 of suitable electrical conductive material such as goldhaving the thickness of about 0.1 micrometers can be evaporated at about60 angle to cover the passivation layer 55 but not touching the tip asis illustrated in FIG. 14. The reflective layer 58 provides means ofenhancing the sensitivity of the wafer to input radiations throughinternal reflection. In addition, the layer 58 provides means ofapplying an electrical potential across the front emitting area of thephotocathode which may be utilized to enhance the field involved at thetop of the emitter projections and also used for gating or modulatingthe emission.

In FIG. 16 there is shown an F-N plot of a typical device. The linearityof this plot at low anode voltages indicates that the emission isFowler-Nordheim limited. The tendency of the curve to saturate for ahigh anode voltages indicates the beginning of the source-limited modeof operation. In this case, the high dark current is being provided bythe high surface and bulk generation in the ungettered and unpassivatedtype assemblies. However even in this low lifetime [nongettered]specimen, a reflection photoemissive sensitivity exceeding 1,500microamperes per lumen was observed. This is practically equivalent tothe value of the 1,650 microamperes per lumen reported in the literaturefor certain lll-V compound emitters. 1t must be emphasized that over50,000 points were simultaneously and uniformly emitting.

H0. 17 indicates the response of the photocathode as described herein bycurve 70 with respect to wavelength of input radiation. Curve 71 is atypical S photocathode and curve 72 is a typical S--l photocathode.

It should be noted that this particular method of fabrication describedherein not only lends itself to the manufacture of transmissive typestructures, that is, where light or energy is directed on one side andemission from the opposite side but also these techniques may beutilized to fabricate devices wherein the illumination is directed ontothe same surface as the electron emission array. 1n addition, thetechniques may be utilized to fabricate conventional cold field emitterswhether made up of semiconductive materials or metals.

1n the specific embodiment illustrated, the mosaic of resist material ismade up of a plurality of circular islands. lt is obvious that theseislands may be of any desirable shape such as square or rectangular. inaddition, the type of crystal orientation definitely affects the densityof the emitter projections and it was found that the (111) and (110)were particularly desirable.

We claim as our invention:

1. A cold cathode field emitter device comprising a single crystallinewafer member of semiconductor material having a minority carrierlifetime of greater than 1 microsecond, said wafer member including asubstrate body member having an array of closely spaced non-growthprojections extending from one surface of said body member, saidsubstrate body member and said projections being crystallographicallycontinuous, and said projections having tip diameters of less than 1.0micron.

2. The device set forth in claim 1 in which said crystalline wafer is asilicon p-type material having a crystallographic orientation of (111).

3. The device set forth in claim 2, in which said silicon p-typematerial has a resistivity of greater than 0.1 ohm centimeter.

4. The structure set forth in claim 1 in which said wafer is providedwith a P+ region on the opposite surface with respect to the surfacefrom which the projections extend.

5. The structure set forth in claim 1 in which a passivating coating isprovided on the opposite surface of said wafer with respect to saidprojection array and a passivation coating is provided on the projectionarray surface intermediate the tips of said projections.

6. The device set forth in claim 1 in which an electrically conductivecoating is provided on the projection array surface of said waferintermediate the emitting tips of said projections.

7. The device set forth in claim 1 in which said body member hasthickness of about 25 micrometers and the height of said projectionemitters is about 12 micrometers.

' 8. The device set forth in claim 1, in which said substrate bodymember has a thickness of from about 25 to 500 micrometers, and saidnon-growth projections extend from said substrate body member for aheight of less than about 50 micrometers whereby charge carriers'createdthroughout the substrate body member will diffuse out to and along saidprojections for emis' sion.

9. The device as set forth in claim 1, wherein the density of saidprojections is about 1.5 X 10 projections per square millimeter.

10. The device specified in claim 1, wherein the semiconductive materialis radiation sensitive.

11. The device set forth in claim 10, in which said radiation sensitivecold cathode field emitter device is utilized as a photocathode elementin an image sensor device.

12. A radiation sensitive cold cathode field emitter device comprising asingle crystalline wafer member of semiconductive material, said wafermember including a substrate body member having a thickness of fromabout 25 to 500 micrometers, and having an array of closely spacedsubstantially impurity free projections extending from the'surface ofsaid body member to a height of less than about 50 micrometers, with theminority carrier lifetime of the device being greater than about onemicrosecond, said substrate body member and said projections beingcrystallographically continuous and said projections having tipdiameters of less than 1.0 micron.

13. The device specified in claim 12, wherein the closely spacedprojections are spaced at about 1.5 X

10 projections per square millimeter.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,81t,968

DATED 1 June 1, 197 4 INVENTOR(5) 1 Harvey C. Nathanson,.Richard N.Thomas and Jens Guldberg It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, lines 7 and 8, cancel "[73] Assignee: Joseph Lucas,(Industries) Limited, Birmingham, England" and substitute [73] Assignee:Westinghouse Electric Corporation, Pittsburgh, Pennsylvania Signed andScaled this second D3) Of March 1976 [SEAL] Attest:

RUTH C. MASON C. MARSHALL DANN Arresting Office Commissioner oj'larentsand Trademarks

2. The device set forth in claim 1 in which said crystalline wafer is asilicon p-type material having a crystallographic orientation of (111).3. The device set forth in claim 2, in which said silicon p-typematerial has a resistivity of greater than 0.1 ohm centimeter.
 4. Thestructure set forth in claim 1 in which said wafer is provided with a P+region on the opposite surface with respect to the surface from whichthe projections extend.
 5. The structure set forth in claim 1 in which apassivating coating is provided on the opposite surface of said waferwith respect to said projection array and a passivation coating isprovided on the projection array surface intermediate the tips of saidprojections.
 6. The device set forth in claim 1 in which an electricallyconductive coating is provided on the projection array surface of saidwafer intermediate the emitting tips of said projections.
 7. The deviceset forth in claim 1 in which said body member has thickness of about 25micrometers and the height of said projection emitters is about 12micrometers.
 8. The device set forth in claim 1, in which said substratebody member has a thickness of from about 25 to 500 micrometers, andsaid non-growth projections extend from said substrate body member for aheight of less than about 50 micrometers whereby charge carriers createdthroughout the substrate body member will diffuse out to and along saidprojections for emission.
 9. The device as set forth in claim 1, whereinthe density of said projections is about 1.5 X 103 projections persquare millimeter.
 10. The device specified in claim 1, wherein thesemiconductive material is radiation sensitive.
 11. The device set forthin claim 10, in which said radiation sensitive cold cathode fieldemitter device is utilized as a photocathode element in an image sensordevice.
 12. A radiation sensitive cold cathode field emitter devicecomprising a single crystalline wafer member of semiconductive material,said wafer member including a substrate body member having a thicknessof from about 25 to 500 micrometers, and having an array of closelyspaced substantially impurity free projections extending from thesurface of said body member to a height of less than about 50micrometers, with the minority carrier lifetime of the device beinggreater than about one microsecond, said substrate body member and saidprojections being crystallographically continuous and said projectionshaving tip diameters of less than 1.0 micron.
 13. The device specifiedin claim 12, wherein the closely spaced projections are spaced at about1.5 X 103 projections per square millimeter.